Background & Significance: Cardiorespiratory fitness (CRF) is an independent predictor of health outcomes in children and is associated with functional limitations in walking/running capacity in obese youth. Obese children are typically less physically active than their healthy-weight peers and are often assumed to be unfit. This study sought to investigate the relationships between adiposity, physical activity levels and CRF in obese and healthy-weight children. Methods: Obese (N=107) and healthy-weight (N=132) 10-13 year olds participated. Fat-free mass (FFM) and percent fat were assessed by dual energy X-ray absorptiometry. Physical activity was assessed by accelerometry and peak oxygen uptake (VO2peak) by maximal cycle ergometry. VO2peak was corrected for mass and FFM. Mean oxygen uptake during unloaded cycle warm-up was also expressed as a percentage of VO2peak (%UVO2peak). Analyses were adjusted for socioeconomic status (SES) and physical activity (accelerometry). Major Findings: Higher percent fat was inversely associated with VO2peak normalized for mass (r=-0.325, P<0.001) and was positively related to %UVO2 (r=0.426, P<0.001) irrespective of SES and physical activity. Higher percent fat was also inversely associated with VO2peak normalized for FFM (r=-0.249 P<0.001, adjusted for SES), but not after controlling for physical activity (r= -0.096 P=0.158). Concluding statement: Higher adiposity, indicative of obesity, is associated with poorer CRF relative to mass irrespective of physical activity levels. However, low physical activity levels may be responsible for associations between adiposity and CRF relative to FFM, indicating the importance of encouraging physical activity in obese youth.